Colloidal quantum dots provide a powerful materials platform to engineer optoelectronics devices, opening up new opportunities in the thermal infrared spectral regions where no other solution-processed material options exist. This mini-review collates recent research reports that push the technological envelope of colloidal quantum dot-based photodetectors toward mid- and long-wavelength infrared. We survey the synthesis and characterization of various thermal infrared colloidal quantum dots reported to date, discuss the basic theory of device operation, review the fabrication and measurement of photodetectors, and conclude with the future prospect of this emerging technology.

Lately discovered silver selenide (Ag2Se) colloidal quantum dots with tetragonal crystal structure exhibit promising optical properties in the mid-wavelength infrared. Although colloidal synthesis of uniform sizes and shapes as well as detailed phase transformation and photoluminescence properties have been studied recently, investigations of their optoelectronic properties as an active layer in photodetector devices remain scarce. Herein, we present the fabrication and characterization of Ag2Se colloidal quantum dot-based photoconductive photodetectors. We investigate the effect of ligand exchange as well as temperature and spectral-dependent photoresponses. Our results suggest that further enhancement in performance could be achieved through accurate control of carrier concentration. With this improvement, Ag2Se colloidal quantum dots may serve as a promising mid-wavelength infrared absorber for the development of thermal infrared sensors and imagers with low size, weight, power consumption, and cost.

In the past 30 years, scientists have utilized quantum confinement to obtain size-tunable interband optical transitions in colloidal quantum dots (CQDs) and implemented them in various optoelectronic applications throughout the electromagnetic spectrum. The infrared (IR) region is particularly important with applications in telecommunications, night-time surveillance, and satellite imaging for agricultural water conservation. Nearly all progress with CQDs in the IR region has been achieved using interband transitions in Pb- and Hg-based heavy metal compounds with narrow band gaps. An alternative approach is to exploit intraband optical transitions originating from external- or self-dopants, which could expand the library of materials for IR-optoelectronic devices to include nontoxic materials. Herein, we present a simple two-precursor hot-injection (170 °C) synthesis of 2.6–6.5 nm diameter environmentally benign Ag2Se CQDs that exhibit a crossover from interband near-infrared (NIR) absorption to intraband mid-wave infrared (MWIR) absorption above a critical size of 5.1 nm. CQDs smaller than 5.1 nm are photoactive in the NIR, exhibiting multiple well-defined excitonic peaks and stable room-temperature emission in the NIR and short-wave infrared (SWIR) regions of the electromagnetic spectrum. Films cast from these CQDs and ligand-exchanged with ethanedithiol exhibit NIR photoconductivity. In contrast, CQDs larger than 5.1 nm exhibit MWIR absorbance. Compared to other synthetic methods that generate Ag2Se CQDs over a limited size range, our approach allows access to both ultrasmall and large Ag2Se CQDs, enabling a detailed study of the size-dependent interband to intraband optical transition. We compare the competing effects of quantum confinement, environmental Fermi level, and particle stoichiometry to provide guidelines for stable electron occupation of the 1Se state and obtain tunable intraband MWIR absorption.

Cu 2 Se thin films provide a promising route toward relatively safe, sustainable and solution processed thermoelectric (TE) modules in contrast to more expensive and toxic materials currently on the market such as Bi 2 Te 3. Cu 2 Se is known in the TE community for its high performance at high temperature and has recently attracted attention from its large theoretically predicted figure of merit at room temperature. Unfortunately, one of the main limitations encountered so far in Cu 2 Se thin films is that the carrier concentrations are not optimized for TE operation after solution processing. In this work, we conduct a comprehensive study of the structural, optical, and TE properties of Cu 2 Se thin films and demonstrate that nonoptimized carrier concentrations in these films lead to observations of poor performance at room temperature. Through a simple soaking procedure in a Cu + ion solution for only a few minutes, we demonstrate a 200− 300% increase in power factor. This soaking process pushes the carrier concentration of the Cu 2 Se thin film toward its optimal value for TE operation and marks the highest TE performance for any solution processed Cu 2 Se thin film at room temperature thus far. If the performance can be further optimized at room temperature, Cu 2 Se thin films will be the material of choice to utilize in TE modules for powering miniature electronics and sensors, which has been an increasingly popular and rapidly expanding market.

A high-performance n-type thermoelectric Ag2Se thin film via cation exchange using a low-cost solution processed Cu2Se template.